Hovercraft or air-cushion vehicles (ACVs) are commonly used to transport passengers and cargo over variable terrain including water, snow, and land. ACVs typically use a skirt system to contain a volume of air, or “air cushion,” which supports the weight of the vehicle during operation. An ACV skirt system commonly includes a skirt bag and multiple smaller cells or “fingers” that are adjacent to one another and located around the lower periphery of the skirt bag. The skirt bag forms a boundary or curtain around the perimeter of the ACV hull to contain the air cushion. The fingers help to form an air cushion seal between the skirt bag and the underlying terrain, e.g., land or water.
FIG. 1 shows a perspective view of the underside of a representative hovercraft or ACV including a prior art skirt system 100 with direction of forward travel 1 toward the ACV bow indicated. The prior art skirt system 100 includes a peripheral bag or skirt bag 106 and fingers 110. The skirt bag 106 is attached to the periphery of the underside of an ACV hull 102. The hull 102 may be attached to a deck 108 as shown. The skirt bag 106 may include bow 106a, port 106b, stem 106c, and starboard 106d sections.
The design of a finger 110 may vary, depending on the location of a particular finger on the perimeter of the skirt bag 106. Different finger designs may help to maintain the air cushion as the ACV moves in various directions and over different types of surfaces and terrain. For example, an open-cone or open-finger design is commonly used for fingers 110a located at the bow section 106a and side sections 106b, 106d of the skirt bag 106. An open finger 110a may include an open loop or cone of flexible material that is attached to the peripheral bag 106 along a perimeter. For further example, a closed-cone or closed-finger design is commonly used for fingers located at the stem and stem corner locations of a skirt system, e.g., skirt system 100. Closed fingers 110b may include a closed loop or cone of flexible material, forming a substantially closed air cell or compartment.
The skirt bag 106 is typically inflated through ports or feed ducts that are supplied with air by one or more fans located on the deck 108. The fingers 110 are typically inflated with air from the skirt bag and/or air from the air cushion. Feed holes in the skirt bag may supply air to closed fingers. Open fingers are open to and receive a supply of air from the air cushion. Open fingers may also have local feed holes supplying air from the skirt bag to facilitate inflation.
FIG. 2 shows a profile of a prior art skirt system 200 including a skirt bag 202 connected to a portion of an ACV hull 206. A finger 204 is attached to the skirt bag 202. The direction of the bow of the ACV is indicated 1. The finger 204 is shown as a conventional closed finger design and would normally be located at the stem or stem corners of a skirt bag. The finger 204 may include a leading face or edge 204a and a swept-back finger or cone portion 204b that is formed by the looped edge of the finger or cone material. The leading edge 204a may face in a desired direction relative to the skirt and ACV hull, depending on the type of finger and its location on the skirt bag 202, e.g., toward the bow 1 or towards the port and starboard directions. The leading edge 204a may form a leading edge angle 208 with the underlying terrain or sea surface 10 as shown. The finger 204 design shown may sometimes be referred to as a “swept-back cone” design, in reference to the finger resembling a cone of fabric or flexible material that has been cut in half.
With continued reference to FIG. 2, the stem finger 204 is attached to the skirt bag 202 at an attachment perimeter 214 (only one portion of the attachment perimeter is shown). One of skill in the art will understand that the attachment perimeter 214 will typically have an open-ended shape for an open finger design, e.g., in the shape of the letter “U” or an open oval shape. For a closed finger design, an aft panel forming a back face 204c would normally be included to seal the swept-back cone portion 204b. 
FIG. 3 shows a profile of the prior art skirt system 200 of FIG. 2 with a conventional closed finger 204 that is in a collapsed position. Closed fingers 204 may collapse in certain situations, such as when a water wave 10 impacts the leading edge 204a of the finger 204. When the pressure and forces on the leading edge 204a exceed the counter pressure and force of the air inside the finger 204, the leading edge 204a can become turned inside out thereby creating a scoop 214, e.g., a concave surface with an included angle, as shown.
The creation of a scoop 214 is sometimes referred to as “scooping”. When scooping occurs in one or more close fingers, large water-generated forces or water loads can result in a degradation in ACV performance and ultimately either a material or attachment structural failure to a finger and/or adjacent skirt structure. These types of failures are sometimes referred to as finger “blow-out”. Accordingly, FIG. 3 illustrates one problem associated with prior art stern fingers, e.g., finger 200.
FIG. 4 shows two profiles, FIG. 4A and FIG. 4B, that show a prior art ACV skirt system 400 with a finger and planing element or stiffener 414. Planing elements or stiffeners, such as 414, have been used to prevent scooping and related problems, described above, for closed stem fingers or stem finger cones in certain applications. Planing element 414 is attached to a leading edge 404a of the closed finger 404. FIG. 4A shows the direction of ACV movement 1 relative to the underlying water or terrain 416 for forward motion of the ACV. FIG. 4B shows the ACV moving in a reversed direction relative to FIG. 4A, with the ACV movement 3 and underlying terrain, e.g., water, movement 4 indicated.
Stiffeners such as 414 may reduce the occurrence of scooping, however they may introduce other disadvantages for the associated skirt systems. Stiffeners are typically made from multiple bonded layers of skirt fabric or hard plastic. As a result of such construction, cones or fingers with stiffeners may be two to three (or more) times heavier than conventional open-finger designs. Additionally, fingers with stiffeners such as 414 may be more expensive than open-finger designs.
With particular reference to FIG. 4B, another notable problem associated with the use of stiffeners 414 may be seen. When an ACV having stem fingers 404 with leading edge stiffeners 414a backs up or moves to the stem, the stiffener tip 414a can snag on the underlying terrain or water surface 416 as shown. Such snagging of the stiffener tip 414a may increase the moving resistance of the ACV and possibly result in damage to the finger 404 and adjacent structure of the skirt system 400. For example, flagellating remnants of a damaged or blown-out finger with stiffeners may cause damage to adjacent fingers or other portions of the associated skirt system.
FIG. 5 shows the underside of a stem corner of prior art ACV skirt system 500 having fingers with stiffening elements. The direction of water surface flow 1 is shown and represents the direction of water flow for normal forward motion of the related ACV. The skirt system 500 has a skirt bag (omitted for clarity) with attached fingers including stem fingers 504 and stem corner fingers 506. The fingers 504, 506 include stiffening elements 504a, 506a located at the leading edges.
When stiffeners 506a are used with stem corner fingers 506, the stiffening elements 506a may twist and under go a displacement 506b, e.g., either up or down, due to the water surface flow 1 relative to the ACV. The twisting of the stiffeners 506a produces drag on the ACV and stress on adjacent fingers, e.g. stem corner fingers 506. Damage may occur to one or more of the adjacent fingers 506 as a result of such twisting and displacement 506b. 
Attempts have been made to reinforce stem corner fingers to withstand twisting stresses produced by the forces on stiffener elements. However, these efforts have had only marginal success at increasing finger life and have significantly increased the cost and weight of the fingers.
What are needed therefore are lightweight, inexpensive stem fingers for an air-cushion vehicle (ACV) that are resistant to scooping and snagging. What are further needed are related methods of manufacturing such stem fingers for use with ACV skirt systems.